Ron,
Since you fill you tank until water comes out of the vent and the system works well until a certain amount of water is drawn from the tank, it seems to me you are creating a vacuum inside the tank. Is your vent line run in such a way that a water lock is created? Do what a previous poster mentioned, remove the water fill cap and see if the problem goes away. If yes, then the vent is the issue.
Another simple test is run a length of hose directly from the inlet side of the pump into a bucket of water, no issues then it's your suction plumbing.
Good luck,
Ken
Since you fill you tank until water comes out of the vent and the system works well until a certain amount of water is drawn from the tank, it seems to me you are creating a vacuum inside the tank. Is your vent line run in such a way that a water lock is created? Do what a previous poster mentioned, remove the water fill cap and see if the problem goes away. If yes, then the vent is the issue.
Another simple test is run a length of hose directly from the inlet side of the pump into a bucket of water, no issues then it's your suction plumbing.
Good luck,
Ken